Wave energy converters (WECs) have been proposed that take advantage of spatially varying pressure differentials (PDs) in a wave field to drive a fluid flow. In order to accurately assess the pressure forcing on PD devices, physics-based relationships between major device parameters and device performance need to be determined. Herein, a transfer function is developed that relates horizontally oriented PD device configurations and wave conditions to the amount of pressure forcing available to the device. Investigation of the transfer function confirms intuitive expectation but also yields surprising results. The transfer function can be applied to a wave spectrum to create a pressure resource spectrum. By manipulating the device length and orientation, an optimal configuration can be found that maximizes the total harnessable pressure resource for a given wave condition or a wave climate. Optimal device lengths for directional seas are longer than those for nondirectional seas, and a wide range of suboptimal configurations yields a reasonable pressure resource. The pressure resource transfer function is a fundamental tool for understanding how horizontal PD WECs work and designing an optimal device.